An optical delay generator can be used for various applications. Time resolved spectroscopy, optical coherence tomography, jitter compensation, microwave photonic devices such as filters and antenna beam formers are some of these applications [1]-[7]. One of the highly interesting applications would be optical buffering which has been considered in the context of optical packet switching or optical burst switching [5]-[7].
Several methods have been introduced to delay the pulses all optically [5]-[17]. These methods can be divided into three main categories. In the first category, the length of the transmission media is changed to produce the desired delay [5]. Changing of the length can be done by increasing the length of the media or by using the circulation of the light in a specific length of the media. Both fiber and integrated solutions can be used to realize these methods. The main benefit of the methods in this category is that they can produce a large range of delay times [5, 7]. Also, they are independent of the incoming signal properties like bandwidth since only the length of the medium is changed.
In so called “Slow Light” techniques the dispersion of the media is tailored to produce the required delay [8]. This can be done via two different methods. The first method is named material based slow light where different properties of the propagation media are changed to add a linear phase to the spectrum of the incoming pulse which is the meaning of the delay in the spectral domain. This can be achieved by dispersion which is induced near resonance e.g. electromagnetically induced transparency (EIT) techniques or stimulated Brillouin scattering (SBS) based slow light. The second method is named structure based slow light in which a linear phase is added to the spectrum of the incoming signal by a specific structure of the components. This method usually uses fiber Bragg Gratings (FBG) to avoid frequency dependent distortions.
In a third category which is sometimes called wavelength conversion/dispersion method, the wavelength of the input pulse is changed. The required delay comes from the wavelength dependent dispersion of the material [8]-[10]. Afterwards the original wavelength is restored. Several variations of this category have been introduced up to a delay of several microseconds.
Another method proposed for delaying optical pulses is called Quasi-Light-Storage (QLS) [11]-[15]. Fiber based and Frequency-to-time conversion based QLS have been proposed up to a delay of several microseconds.
The performance of each of the above-mentioned schemes can be evaluated by several metrics such as maximum achievable delay, ability to be integrated, size, easy implementation, accuracy of control, the speed of operation, structural complexity, power consumption, and ability for easy tuning of the delay:
The tunability of the first category is in the coarse range, and usually the attenuation of the incoming signal is large.
Delay production methods based on the second category usually cannot produce large amounts of the delay-bandwidth products, e. g. 4 bits for SBS based slow light.
Structure based slow light methods can produce higher amounts of delay, but higher order phase distortions (like dispersion) limit the useful bandwidth of these methods.
The third category requires tunable lasers and filters for the wavelength shift which makes it very complex and power consumptive.